Coil spring, and method for manufacturing same

ABSTRACT

To provide a coil spring having excellent fatigue resistance. 
     Disclosed is a coil spring made of steel, including (in % by mass, the same shall apply for a chemical composition): C: 0.40 to 0.70%; Si: 1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to 1.00%, and Al: 0.01% or less (excluding 0%), with the balance being iron and inevitable impurities, wherein an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency, and wherein a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or higher at a position in a depth of (¼)×diameter in the depth direction from the surface.

TECHNICAL FIELD

The present invention relates to a coil spring and a method for manufacturing same, and more particularly to a coil spring with excellent fatigue resistance and a method for manufacturing same.

BACKGROUND ART

Coil springs are used as a valve spring, a clutch spring, a suspension spring, etc., in the engine, clutch and suspension of an automobile and the like. A coil spring is repeatedly used under a high stress over a long period of time and thus required to have a high level of fatigue resistance.

Wires for the valve spring in the engine as specified by JIS include, for example, an oil-tempered wire for a valve spring (SWO-V: JIS G 3561), a chrome-vanadium steel oil-tempered wire for a valve spring (SWOCV-V: JIS G 3565), and a silicon-chromium steel oil-tempered wire for a valve spring (SWOSC-V: JIS G 3566). In particular, SWOSC-V has been mainly used because of its excellent fatigue resistance.

These wires are produced by drawing rolled wire rod, followed by quenching and tempering, resulting in the desired strength. A general method for manufacturing a valve spring involves coiling such a wire into a spring having a required shape, followed by nitriding, shot peening, tempering, setting, and the like, thereby producing a spring with excellent fatigue resistance.

From the viewpoint of conservation of the environment and natural resources, there have been increasing demands for cleaning exhaust gas from automobiles and improving the fuel efficiency thereof. The reduction in weight of automobiles significantly contributes to these demands, and thus many attempts have been continuously undertaken to reduce the weight of components included in the vehicle body.

The valve spring is modified to improve its fatigue resistance, making the valve spring more compact, which can further contribute to the reduction in weight of the engine. For this reason, some proposals have been presented to improve the fatigue resistance of the valve spring.

For example, Patent Document 1 discloses the technical feature in which a coil spring is designed to have on its surface a carburized layer (of 0.05 to 1.00 mm in depth) with a predetermined composition and also to exhibit the hardness in a depth of 0.02 mm from its surface within a predetermined range (of 650 to 1,000 HV), thereby improving the fatigue resistance.

CONVENTIONAL ART DOCUMENT Patent Document

Patent Document 1: JP 2012-77367 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The coil spring mentioned in Patent Document 1 has a fatigue resistance of a level of fifty million times. In recent years, along with the progress in reducing the weight and enhancing the output of the vehicle, coil springs have been required to exhibit more excellent fatigue resistance.

The present invention has been made in light of the foregoing circumstance, and it is an object of the present invention to provide a coil spring with excellent fatigue resistance and a method for manufacturing such a coil spring with the excellent fatigue resistance.

Means for Solving the Problems

The present invention that can solve the foregoing problems provides a coil spring made of steel, including (in % by mass, the same shall apply for a chemical composition): C: 0.40 to 0.70%; Si: 1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to 1.00%, and Al: 0.01% or less (excluding 0%), with the balance being iron and inevitable impurities, wherein an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency, and wherein a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or more at a position in a depth of (¼)×diameter in the depth direction from the surface.

Further, in a preferred embodiment, the chemical composition of the above-mentioned coil spring comprises: Ni: 1.50% or less (excluding 0%) , and/or Nb: 0.50% or less (excluding 0%).

To manufacture the coil spring with excellent fatigue resistance as mentioned above, it is recommended to perform the vacuum carburization process at 1,000° C. or higher.

Effects of the Invention

Accordingly, the present invention can control the carburizing depth of the surface of the coil spring and the Vickers hardness thereof as appropriate, while properly controlling the chemical composition and the prior austenite crystal grain size, thereby producing the coil spring with excellent fatigue resistance. Furthermore, the method according to the present invention can provide the coil spring with excellent fatigue resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory diagram of measurement positions of a carburized layer in a coil spring as well as measurement positions for Vickers hardness located at a distance of ¼×the diameter.

FIG. 2 is a schematic explanatory diagram of measurement positions for prior austenite crystal grain size of the coil spring.

MODE FOR CARRYING OUT THE INVENTION

The inventors have studied from various perspectives to provide a coil spring that has improved fatigue resistance, compared to the related art, and exhibits excellent fatigue resistance to achieve the result of a fracture-lifetime test that exceeds sixty million times in examples to be mentioned later. In the technique mentioned in Patent Document 1, a metallographic structure is controlled while increasing the amount of added C. However, only this technique did not attain a level of sixty million times of the fracture lifetime (see Example 4 mentioned in Patent Document 1 as well as a test specimen No. 8 in Table 2 obtained by simulating this example).

Thus, to achieve the more excellent fatigue resistance, the inventors have studied chemical compositions, metallographic structures, and the like. As a result of intensive studies, it has been found that as the toughness and strength of the coil spring affect fatigue fracture of the coil spring in use, these factors can be controlled as appropriate to enable drastic improvement of the fatigue resistance of the coil spring.

First, to enhance the strength of the coil spring, it is necessary to ensure some carburized layer depth from the surface of steel configuring the coil spring (hereinafter simply referred to as a “surface”), as well as the adequate Vickers hardness of the inside of steel (which can sometimes be represented by “¼×D”, which means multiplication of a diameter D of a steel wire forming the coil spring by ¼). To sufficiently ensure the depth of the carburized layer as well as the Vickers hardness, it is required to enhance the temperature of the carburization process. Only the carburization process at a high temperature was not able to improve the fracture lifetime of the coil spring. This is because the carburization process at high temperatures coarsens the crystal grains of the prior austenite, or leads to variations in crystal grain size of the prior austenite (which means the presence of a difference in crystal grain number; hereinafter referred to as duplexed grains), drastically reducing the toughness of the coil spring, which might degrade the fracture lifetime.

Regarding these problems, the inventors have diligently investigated and, as a result, found that the chemical composition of the steel can be appropriately controlled to solve the above-mentioned problems. In particular, it has been revealed that by increasing the V content in the chemical composition, the crystal grain of prior austenite is prevented from being coarsened even after the carburization process at a high temperature, further suppressing generation of duplex-grains.

The present invention has been made based on the findings that, on the assumption that the following chemical composition is satisfied, the carburized layer depth, the Vickers hardness, and the prior austenite crystal grain size are appropriately controlled to enable keeping balance between the strength and toughness required for improving the fatigue resistance, thereby providing a coil spring with the excellent fatigue resistance mentioned above.

The chemical composition of the coil spring in the present invention will be described below.

C: 0.40 to 0.70%

Carbon (C) is an element that is effective in ensuring the adequate strength of a coil spring used under a high load and the Vickers hardness of the coil spring in the position of ¼×D. To exhibit these effects, the C content is 0.40% or more, preferably 0.45% or more, and more preferably 0.50% or more. Any excessive C content, however, degrades the toughness of the coil spring and increases surface flaws of the coil spring, resulting in reduced fatigue resistance. Accordingly, the C content should be 0.70% or less, preferably 0.65% or less, and more preferably 0.60% or less.

Si: 1.50 to 3.50%

Silicon (Si) is an element that is effective in ensuring the adequate Vickers hardness, similar as C. Further, Si is also effective in improving the strength of the coil spring, the fatigue resistance and the sagging resistance. To exhibit these effects, the Si content is 1.50% or more, preferably 1.80% or more, and more preferably 2.10% or more. Any excessive Si content, however, degrades the toughness of the coil spring and reduces the cold workability and the hot workability during the manufacturing procedure for the coil spring, which leads to poor yield of products and assists in decarburization due to a heat treatment, thus degrading the fatigue resistance. Accordingly, the Si content should be 3.50% or less, preferably 3.30% or less, and more preferably 3.10% or less.

Mn: 0.30 to 1.50%

Manganese (Mn) is an element that is effective in improving the strength of the coil spring by enhancing the quenching properties. Further, Mn serves to fix, in the steel, sulfur (S) that would adversely affect the fatigue resistance, to thereby convert it into MnS, which reduces the above-mentioned disadvantage. To exhibit these effects, the Mn content is 0.30% or more, preferably 0.40% or more, and more preferably 0.50% or more. Any excessive Mn content, however, degrades the toughness of the coil spring and also reduces the cold workability and the fatigue strength. Accordingly, the Mn content should be 1.50% or less, preferably 1.20% or less, and more preferably 0.90% or less.

Cr: 0.10 to 1.50%

Chromium (Cr) is an element that is effective in improving the strength of the coil spring by enhancing the quenching properties, similar as Mn. Cr also has the effects of reducing activity of C to prevent the decarburization in the hot-rolling process or the heat treatment. To exhibit these effects, the Cr content is 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. Any excessive Cr content, however, drastically decreases a C diffusion coefficient in a vacuum carburization process, making it difficult to form a desired carburized layer, resulting in reduced fatigue resistance. When the carburization temperature is increased to ensure the desired carburized layer, the prior austenite crystals are coarsened while generating duplex-grains, thus degrading the fatigue resistance of the coil spring. Accordingly, the Cr content should be 1.50% or less, preferably 1.20% or less, and more preferably 0.90% or less.

V: 0.50 to 1.00%

Vanadium (V) is an element that is effective in making the prior austenite crystal grains finer. Especially, V is the element that is also effective in suppressing the coarsening of the prior austenite crystal grains and generation of duplex-grains, which are problems in the related art when the carburization temperature is increased to ensure the desired carburized layer. To exhibit these effects, the V content is 0.50% or more, preferably 0.53% or more, and more preferably 0.56% or more. Any excessive V content, however, forms a large amount of V carbide, degrading the ductility, the cold workability, and the resistance to the fatigue of the coil spring. Accordingly, the V content should be 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

Al: 0.01% or less (excluding 0%)

Aluminum (Al) is a deoxidizing element but any excessive Al content forms inclusions, such as AlN. These inclusions drastically degrade the fatigue resistance of the coil spring. Accordingly, the Al content needs to be reduced to 0.01% or less, preferably 0.008% or less, and more preferably 0.006% or less.

The basic chemical composition of the steel configuring the coil spring in the present invention has been mentioned above, with the balance being substantially iron. Here, the term “substantially” as used herein means that the present invention allows, without departing from the feature of the invention, the contamination of a very small amount of elements present in a steel raw material, including scraps, and which would inevitably occur during an iron manufacture process, a steel manufacture process, further, a steel-manufacture preliminary treatment process, and the like. For example, exemplary inevitable impurities include P (preferably, of 0.016% or less, and more preferably 0.015% or less), and S (of 0.015% or less).

The invention may contain both or either of Ni and Nb in the following ranges as other elements, as needed. The characteristics of the coil spring are further improved depending on the kinds of contained elements.

Ni: 1.50% or less (excluding 0%)

Nickel (Ni) is an element that is effective in improving the toughness of the coil spring that increased its strength by C. To exhibit these effects, the Ni content is preferably 0.05% or more, and more preferably 0.10% or more. Any excessive Ni content, however, generates residual austenite in an excessively amount, which degrades the fatigue resistance of the coil spring. Accordingly, the Ni content is preferably 1.50% or less, more preferably 1.20% or less, and much more preferably 0.90% or less.

Nb: 0.50% or less (excluding 0%)

Niobium (Nb) has the effect of making the crystal grains finer in the hot-rolling process as well as the quenching-and-tempering process, thereby improving the ductility of the coil spring. To exhibit these effects, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more. Any excessive Nb content, however, generates the V carbides in an excessive amount to thereby degrade the ductility of the coil spring, reducing the cold workability and the fatigue strength. Accordingly, the Nb content is preferably 0.50% or less, more preferably 0.40% or less, and much more preferably 0.30% or less.

To improve the fatigue resistance, it is important to appropriately control not only the chemical composition as mentioned above, but also the metallographic structure (control of the prior austenite crystals), the carburized layer and the Vickers hardness of the steel of the coil spring.

Average crystal grain size number of the prior austenite crystals: 11.0 or more

The crystal grain size of the prior austenite crystals in a depth of 0.3 mm from the surface of the coil spring can be made finer to increase its crystal grain size number, thereby enhancing the toughness thereof to drastically improve the fatigue resistance of the coil spring. To exhibit these effects, the average crystal grain size number of the prior austenite crystal is 11.0 or more, preferably 12.0 or more, and more preferably 13.0 or more. From the viewpoint of improving the toughness, the upper limit of the average crystal grain size number of the prior austenite crystal is not specifically limited. However, in terms of the easiness of manufacturing and the cost of alloys, the average crystal grain size number is preferably approximately 15.0 or less, and more preferably 14.0 or less.

Difference in grain size number between the prior austenite crystals: within a range of less than 3 from the grain size number observed at the maximum frequency

When variations in crystal grain size number of the prior austenite crystals measured in a depth of 0.3 mm from the surface are large, the toughness of the coil spring is significantly degraded even though the prior austenite crystals in the steel of the coil spring satisfy the above-mentioned average grain size number, which makes the cold workability and the fatigue resistance worse. Therefore, in the present invention, the measured crystal grain size number of each prior austenite crystal needs to be within a difference of less than 3, preferably 2 or less, and more preferably 1 or less from the grain size number observed at the maximum frequency. Note that in the present invention, the state in which such a condition for the difference in grain size number is satisfied is referred to as “no duplex-grain”.

In the present invention, the austenite crystal grains in the steel wire of the coil spring satisfy the above-mentioned average crystal grain size number, and further the formation of duplex-grains is suppressed, whereby the fatigue resistance can be improved.

Carburized layer: in a depth of 0.30 to 1.00 mm from the surface of the coil spring

The appropriate carburized layer is effective in improving the fatigue resistance of the coil spring. That is, the surface side of the coil spring is sufficiently hardened, which can suppress the occurrence of fracture starting from the surface of the spring when the coil spring is repeatedly used under the high load stress. To exhibit these effects, the carburized layer needs to be formed in at least a depth of 0.30 mm or more, preferably 0.40 mm or more, and more preferably 0.50 mm or more, from the surface of the coil spring. However, when the rate of the carburized layer in the steel wire of the coil spring becomes excessive, coarsened carbides are precipitated, which might degrade the fatigue resistance of the coil spring. Therefore, the carburized layer needs to be formed in a depth of 1.00 mm or less, preferably 0.90 mm or less, and more preferably 0.80 mm or less from the surface of the coil spring.

Average Vickers hardness in the position of (¼)×diameter D in the depth direction from the surface: 600 or more

The coil spring formed of steel, the inside of which has an appropriate Vickers hardness (Hv), is effective in improving the fatigue resistance of the coil spring. Specifically, when the inner hardness of the coil spring is low, in repeatedly use of the coil spring under a high load stress, plastic deformation occurs in the coil spring even though the stress applied to the spring is below a limit of elasticity. As a result, the required spring stress cannot be exhibited, degrading the fatigue resistance of the coil spring. Thus, from the viewpoint of improving the fatigue resistance, an average Vickers hardness at least in the depth (¼)×D from the surface of the coil spring is 600 or more, preferably 670 or more, and more preferably 690 or more. The upper limit of the average Vickers hardness is not specifically limited. However, when the Vickers hardness is too high, the toughness of the coil spring would be reduced, thus degrading the fatigue resistance. Accordingly, the above-mentioned average Vickers hardness is preferably 750 or less, and more preferably 730 or less.

When manufacturing the coil spring with the excellent fatigue resistance as mentioned above, manufacturing conditions therefor can be desirably controlled as appropriate. In particular, to ensure the above-mentioned predetermined carburizing depth and Vickers hardness (average thereof), it is effective to control the temperature of the vacuum carburization process. Preferable conditions for manufacturing the coil spring in the present invention will be described below.

The coil spring in the present invention can be manufactured by subjecting a steel material satisfying the above predetermined chemical composition to melting, hot forging, and hot rolling into a wire rod having a desired wire diameter, followed by shaving, patenting, wire-drawing and oil tempering of the wire rod, and thereafter forming the obtained wire into a spring, which is then subjected to vacuum carburization process. Thereafter, to further improve the fatigue properties, shot peening, setting, or the like may be performed as needed.

The conditions for the aforesaid melting, hot forging and hot rolling are not specifically limited, and thus may be conventional manufacturing conditions. For example, a steel ingot satisfying the above predetermined chemical composition is manufactured through melting in a blast furnace, and the steel ingot is subjected to blooming to produce a billet with a predetermined size. In order to suppress the deformation resistance that would affect the workability as well as the coarsening of the prior austenite crystal grains, the billet might be heated, for example, to approximately 900° C. to 1100° C., and then hot-rolled at a desired rolling reduction to form a wire rod with a desired shape property. Thereafter, a deoxidized layer formed on the surface of the wire rod is removed by being shaved by a desired thickness. To remove the processed hardened layer generated by the shaving process and to obtain desired micro structure (for example, pearlite) with excellent drawability, the patenting process, or a soft annealing process or the like in an IH (induction heating) equipment is performed.

Thereafter, the wire rod is drawn into one with a desired wire diameter, followed by the oil tempering process to thereby form a wire for a spring. Then, the wire is formed into a spring with the desired coil diameter, free height and number of turns. The reason why the wire is formed into the spring shape before the carburization process is that after the carburization quenching and tempering for forming the carburized layer, the surface part of the steel (carburized layer) becomes hard and the ductility of the wire is degraded, making it difficult to form the coil spring.

After forming the spring shape, the vacuum carburization process is performed. However, in the present invention, to attain the predetermined carburizing depth and Vickers hardness, the vacuum carburization process needs to be performed at a high carburization temperature of 1,000° C. or higher. When the carburization temperature is lower than 1,000° C., the desired carburized layer and Vickers hardness cannot be obtained, which degrades the fatigue resistance. The carburization temperature is preferably 1,020° C. or higher, and more preferably 1,040° C. or higher. When the carburization temperature becomes too high, however, the carbides are coarsened and precipitated, so that the coil spring becomes excessively hard, reducing its toughness, which may lead to degradation of the fatigue resistance. The carburization temperature is preferably 1,100° C. or lower, and more preferably 1,080° C. or lower.

Then, the carburization process is applied to the coil spring. As the degree of decarburization is increased during the carburization process, or as variation in processing temperature becomes larger, the fatigue strength of the coil spring is degraded. Thus, in the present invention, the vacuum carburization process is performed from the viewpoint of suppressing the decarburization and temperature variation. The vacuum carburization process is performed at a temperature of 1,000° C. or higher, whereby the carburized layer can be uniformly formed in the desired thickness. The carburization time and the diffusion time are not specifically limited and may be any adequate times that form the desired carburized layer. For example, the carburization time may be set at 1 to 10 minutes, and the diffusion time may be set at 1 to 10 minutes.

After the carburization process, gas cooling or oil quenching is continuously performed down to a temperature of the A_(l) transformation temperature or lower. Then, a re-heating process (for example, at a temperature of 830° C. to 850° C. for 10 minutes to 30 minutes) is desirably performed, whereby the prior austenite crystal grains can be made much finer. The tempering process may be performed to improve the toughness and ductility as needed.

The obtained coil spring may be subjected to the conventional shot peening and setting as appropriate for the purpose of further improving the fatigue resistance of the coil spring.

When manufacturing the coil spring of the present invention, any conditions other than the above-mentioned ones are not specifically limited, and general manufacturing conditions may be applied.

The coil spring obtained in this way can be used as the coil spring with excellent fatigue resistance in various applications, including a valve spring for an engine, a spring for a transmission, and the like, as mentioned above.

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-143514, filed on Jul. 9, 2013, the entire contents of which are incorporated herein by reference.

EXAMPLES

The present invention will be described in more detail below by way of Examples. It should be noted that, however, these examples are never construed to limit the scope of the invention; various modifications and changes may be made without departing from the scope and spirit of the invention and should be considered to be within the scope of the invention.

A steel material was melted in a vacuum melting furnace to form steels A to H having the chemical compositions shown in Table 1 below (with the balance being iron and inevitable impurities) and subjected to hot forging, thereby fabricating billets of 155 mm square. Each billet was heated at 1,000° C. and hot-rolled to produce a wire rod for a spring having a diameter of 8.0 mm. The wire rod for a spring was subjected to the soft annealing (while being kept at 660° C. for 2 hours), and then a surface part of the wire rod for a spring was shaved by 0.15 mm to thereby remove a decarburized layer. Thereafter, the wire rod for a spring was heated at a temperature of 900° C. or higher in a neutral gas atmosphere to thereby be austenitized. Then, a lead-patenting process (heating temperature: 980° C., lead furnace temperature: 620° C.) was performed on the wire rod for a spring to take place pearlite transformation. Thereafter, the wire rod for a spring was subjected to the cold drawing into a wire having a diameter of 4.1 mm, and then to the oil tempering process under the conditions appropriate for the respective components of the wire (heating temperature: 900° C. to 1,000° C., quenching oil temperature: 60° C., tempering temperature: 400 to 500° C.), thus fabricating the wire for a spring. The wire for a spring was used to be cold-formed, thereby producing a spring (average coil diameter: 24.60 mm, free height: 46.55 mm, the effective number of turns: 5.75).

Then, the thus-obtained spring was heated to the “carburization temperature” mentioned in Table 2 below and subjected to the vacuum carburization process (carburization time: 5 minutes, diffusion time: 3 minutes). Thereafter, the spring was kept at 950° C. for 15 minutes, and immersed in oil kept at 50° C. to be quenched, followed by being tempered (at 350° C., for 90 minutes). The thus-obtained spring was subjected to three-stage shot peening (by gradually decreasing a diameter of a shot particle from the first stage), and subsequently hot setting (at 230° C., T_(max)=1,600 MPa-equivalent). The following measuring and tests were performed on the coil springs obtained in this way (test samples Nos. 1 to 13).

(Carburized Layer Depth)

The carburized layer depth in each test sample was determined by measuring the carbon content in the coil spring. Specifically, as shown in FIG. 1, four lines were drawn at intervals of 90 degrees from the center of the cross section of the steel wire forming the coil spring in each test sample. Then, the depth on each line in which the carbon (C) content (%) became substantially equal to that of carbon added to the steel was measured in the sample. Measurements in this test were shown in the “carburized layer depth” column in the table. In the present invention, the samples having the carburized layer depth in a range from 0.30 mm to 1.00 mm were rated as “pass”.

(Vickers Hardness in ¼×D Position)

The hardness (Hv) of the coil spring in each test sample was measured using the Vickers hardness tester. Specifically, as shown in FIG. 1, the hardness of the coil spring in each sample was measured on the four lines drawn at intervals of 90 degrees from the positions at ¼×diameter D (d/4) of the cross section of the steel wire that forms the coil spring in each test sample (test load: 10 kgf). Then, an average of these measurements was determined. The average Vickers hardness was shown in the “Vickers hardness” column. In the present invention, the samples having the Vickers hardness of 600 or more were rated as “pass”.

(Average Crystal Grain Size Number of Prior Austenite Crystals)

A method for measuring a crystal grain size of the prior austenite crystals in the coil spring was as follows. Specifically, first, as shown in FIG. 2, the cross section of the coil spring in each sample was partitioned at intervals 45 degrees from the center thereof into eight regions. In the respective regions, the crystal grain size of the prior austenite crystal in the depth of 0.3 mm from the surface of the steel wire forming the coil spring in the direction to the center thereof was observed and measured in each test sample with an optical microscope (at 400-fold magnification) in accordance with JIS G 0551 (size of the field of view: 250 μm×200 μm). The average of measurements was shown in the “average crystal grain size number of prior γ-crystal” column. In the present invention, the samples having the average crystal grain size number of the prior austenite crystals of 11.0 or more were rated as pass.

(Difference in Grain Size Number between Prior Austenite Crystals)

A method for determining a difference in grain size number between the prior austenite crystals in the coil spring was as follows. Some samples had the above-mentioned measured crystal grain size number of the prior austenite crystal that differed by three or more from the grain size number observed at the maximum frequency. These samples were determined to contain duplex-grains. The samples where the duplex-grain was present were defined as “present” in the “Duplex-grain” column of the table, while the samples where the duplex-grain was not present were defined as “not present”.

(Fatigue Resistance: Fatigue Test)

A shear stress with the maximum shear stress (T_(max)) of 588±441 MPa was applied to each test sample obtained in the above way, and the test sample was subjected to the fatigue test up to sixty million times. The test samples to which the shear stress could be applied sixty million times (that is, which were not broken) were defined as the “A” determination (which means excellent fatigue resistance), and then these samples were shown as “>6000” in the table. The test samples to which the shear stress could not be applied sixty million times (that is, when the test sample was broken midway) were defined as the “F” determination (which means the failure of the test, or inferior fatigue resistance), and then the number of application of the shear stress that caused the breakage of the sample was recorded in the table.

TABLE 1 Chemical composition (% by mass) Steel type C Si Mn Cr V Al Ni Nb P S A 0.47 2.96 0.53 0.62 0.56 0.003 0.00 0.00 0.010 0.006 B 0.55 3.03 0.52 0.60 0.73 0.003 0.00 0.00 0.012 0.008 C 0.55 3.04 0.52 0.21 0.72 0.006 0.00 0.00 0.008 0.010 D 0.46 2.21 0.71 0.30 0.66 0.002 0.02 0.00 0.016 0.009 E 0.60 1.98 0.55 0.62 0.75 0.003 0.00 0.02 0.011 0.008 F 0.60 2.10 0.50 1.76 0.26 0.004 0.20 0.00 0.013 0.006 G 0.59 2.10 0.51 1.30 0.44 0.003 0.20 0.00 0.011 0.007 H 0.37 1.20 0.35 0.70 0.63 0.004 0.00 0.00 0.009 0.009

TABLE 2 Average crystal grain size Fracture Carburization Carburized Vickers number of lifetime Steel temperature layer depth hardness prior [×10⁴ Fracture-lifetime No. type [° C.] [mm] Hv γ-crystal Duplex-grain times] determination 1 A 1,050 0.80 679 11.1 Not present >6,000 A 2 B 1,050 0.65 718 13.6 Not present >6,000 A 3 C 1,050 0.70 690 13.1 Not present >6,000 A 4 D 1,050 0.65 688 13.6 Not present >6,000 A 5 E 1,050 0.75 691 13.6 Not present >6,000 A 6 E 1,030 0.70 685 13.6 Not present >6,000 A 7 E 1,070 0.80 693 13.1 Not present >6,000 A 8 F 850 0.27 681 13.0 Not present 5,000 F 9 F 1,050 0.41 705 10.1 Present 2,000 F 10 G 1,050 0.50 705 11.6 Present 1,000 F 11 H 950 0.43 568 12.1 Not present 2,000 F 12 A 900 0.25 680 12.6 Not present 1,500 F 13 B 900 0.27 708 13.6 Not present 2,000 F

These results can be explained by the following consideration. Samples Nos. 1 to 7 are examples that met the requirements defined by the present invention (chemical composition, crystal grain size, carburized layer depth, and Vickers hardness). All the coil springs of samples Nos. 1 to 7 are found to have a long fracture lifetime with a high load applied thereto (A determination) and an excellent fatigue resistance.

In contrast, samples Nos. 8 to 13 did not satisfy the requirements defined by the present invention, including the chemical composition and the preferable manufacturing conditions, and thus could not ensure the predetermined crystal grain size, carburizing depth, Vickers hardness, and the like, leading to the result of inferior fatigue resistance (F determination).

Samples Nos. 8 and 9 are examples in which the same type of steel was used. These are the examples simulating Example No. 4 disclosed in Patent Document 1 (steel type of A and carburization condition L in Patent Document 1). Samples Nos. 8 and 9 are the examples in which the amount of added V was small, and the amount of added Cr was large. Since the diffusion coefficient of C was drastically reduced, the carburized layer was shallow. In particular, in sample No. 8, the carburization temperature was low, so that the adequate carburizing depth could not be obtained, resulting in worse fatigue resistance. Although in sample No. 9, the processing was performed at the carburization temperature recommended by the present invention, the amount of added V was small, which could not exhibit the sufficient effect of making the crystal grains of the prior austenite crystals finer. As a result, the duplex-grains were generated to degrade the fatigue resistance.

In sample No. 10, since the amount of added V was small, the processing at the predetermined carburization temperature generated duplex-grains, thus degrading the fatigue resistance.

Sample No. 11 is an example in which the amounts of added C and Si were small, and thus the carburization temperature was low. In this example, the predetermined Vickers hardness was not obtained, thereby degrading the fatigue resistance.

In samples Nos. 12 and 13, the carburization temperature was low, whereby the predetermined carburizing depth was not obtained, degrading the fatigue resistance. 

1. A coil spring made of steel, comprising: C: 0.40 to 0.70% by mass; Si: 1.50 to 3.50% by mass; Mn: 0.30 to 1.50% by mass; Cr: 0.10 to 1.50% by mass; V: 0.50 to 1.00% by mass; Al: 0.01% by mass or less (excluding 0%), with the balance being iron; and inevitable impurities, wherein: an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency; and a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or higher at a position in a depth of (¼)×diameter in the depth direction from the surface.
 2. The coil spring according to claim 1, further comprising: Ni: 1.50% by mass or less (excluding 0%), and/or Nb: 0.50% by mass or less (excluding 0%).
 3. A method for manufacturing the coil spring of claim 1, the method comprising performing a vacuum carburization process at 1,000° C. or higher.
 4. A method for manufacturing the coil spring of claim 2, the method comprising performing a vacuum carburization process at 1,000° C. or higher. 